The present invention relates to an exhaust gas purification system that purifies particulate matters (PM) from the exhaust gas discharged by diesel and other internal combustion engines using a continuous regeneration-type diesel particulate filter (DPF) and also to a control method thereof.
In the same way as for NOx, CO, and also HC etc., restrictions on the volume of particulate matters (hereinafter “PM”) discharged from diesel internal combustion engines grow severe every year. Techniques for collecting this PM in a filter known as a diesel particulate filter (hereinafter “DPF”) and for reducing the quantity thereof by discharging externally have been developed. Continuous regeneration DPF devices represent such.
However, even in continuous regeneration DPF devices, increasing exhaust gas pressure as a result of filter clogging has become a problem. In other words, although the PM collected by these continuous regeneration DPF devices is continuously burned and purified, and the filter will self-regenerate while the exhaust gas temperature is approximately 350° C. or greater, when the exhaust gas temperature is low or the traveling condition of an internal combustion engine has a low NO discharge—for example, when an internal combustion engine has a continuously low exhaust gas temperature accompanied by idling or low load/low speed traveling, etc.—the temperature of the exhaust gas drops and the catalyst becomes inactive as a result of the low temperature thereof. Accordingly, as the oxidization reaction does not progress and NO becomes insufficient, the above-described reaction does not take place, and the filter does not regenerate by oxidizing the PM. Accordingly, the accumulation of PM in the filter continues and clogging thereof is accelerated.
Countermeasures for clogging of the filter such as those described below are disclosed in Japanese patent application Kokai publications No. 2002-276340 and No. 2003-206723. Before clogging of the filter has exceeded a predetermined level using methods for detecting the differential pressure before and behind the filter etc. has been detected, if the exhaust gas temperature is lower than the active temperature of the oxidation catalyst provided upstream of the filter or the oxidation filter supported on the catalyst, exhaust gas temperature is forcibly raised and the collected PM then forcibly removed through being burned.
Furthermore, injection control within the cylinders can be used as a means for raising the exhaust gas temperature. In accordance with this method, multi-step injection is carried out and the exhaust gas temperature is raised; post injection is carried out when the temperature thereof has risen beyond the active temperature of the oxidation catalyst; and any unburned fuel contained in the exhaust gas is burned in the oxidation catalyst. As a result of this burning, the exhaust gas temperature is raised beyond the temperature of burning temperature of the PM collected in the filter, and the collected PM removed through the burning thereof, thus regenerating the filter.
Normally with continuous regeneration DPF devices, when the collected quantity of PM reaches a preset limit, the traveling condition of the internal combustion engine is automatically changed to regeneration mode. In this forced regeneration mode operation, the exhaust gas temperature is forcibly raised and the quantity of NOx is increased. And thereby, the collected PM is oxidized and removed from the filter to regenerate the filter.
Furthermore, Japanese patent application Kokai publication No. 2003-155914 proposes an operating means whereby, an actuation means such as a forced regeneration means which can be actuated arbitrarily when a large quantity of particulate matters (PM) has accumulated in the diesel particulate filter (DPF) by some reasons, the driver can intentionally and immediately carry out the forced regeneration of the particulate filter. More specifically, this actuation means is comprised of a warning light that indicates excessive collecting condition and a regeneration button in the vicinity of the driver's seat so that forced regeneration can be arbitrarily controlled.
Furthermore, Japanese patent application Kokai publication No. 2003-155914 proposes to indicate a warning to carry out a human-initiated forced regeneration in which the collected particulate matters are forcibly burned and removed when it is judged that the particulate filter has become clogged upon a confirmation of an abnormal increase in back pressure based on the differential pressure before and behind the particulate filter.
In order to address the problem of oil dilution upon forced regeneration while traveling and the problem of uneven accumulating around the outer surface of the PM filter, a regeneration control method comprised of a combination of manual regeneration and traveling automatic regeneration in response to a travel distance is under consideration.
In terms of oil dilution, it is undesirable that forced regenerating processing be too frequently carried out. If forced regenerating processing is carried out while the vehicle is being traveled, the engine speed will be high in comparison with that of stationary idling; accordingly, it is inevitable that post injection volume increases with a result that the degree of diluting of oil by fuel increases. In particular, a post injection control becomes difficult during transient condition. In other words, even if the engine temperature is raised in a transient condition resulting from load variation, post injection is performed wastefully. It is difficult to avoid this type of wasteful injection. And furthermore, if left unaddressed, oil dilution can lead to the problems of excessive wear or scorching of the moving mechanical components.
In contrast, the forced regeneration control does not cause such problems in a stationary condition of the vehicle and the degree of oil dilution relatively insignificant. As a result, the forced regeneration control is not carried out while the vehicle is being traveled, but after it has stopped traveling.
In other words, when the operating condition is in a stabilized mode such as an idling condition while the vehicle is stationary, the post injection of fuel injection into the cylinder is carried out with an injection quantity corresponding to a load less than that necessary for normal vehicle traveling. As a result, forced regeneration is carried out by raising of the exhaust gas temperature, and oil dilution can be reduced to a level lower than that corresponding to regeneration control while the vehicle is being traveled.
One method thereof is disclosed as follows in Japanese patent application Kokai publication No. 2003-155914. When the filter reaches a preset level of clogging, the driver is notified of the need to carry out forced regeneration using a lamp or such like, and after stopping the vehicle upon receiving of this notification, the driver operates a manual regeneration switch provided in the vicinity of the driver's seat to perform forced regeneration control and filter regeneration.
Furthermore, since a vehicle has a wide range of traveling patterns, problematic clogging unrelated to differential pressure and resulting from uneven loading of PM can also occur. For example, there are many occasions on which vehicles are frequently traveled on high-speed motorways and at high-speed with high-load, and since the exhaust gas temperature is also high, forced regeneration control is not carried out and self-regeneration is urged. However, PM does not accumulate at the center of the filter, and uneven accumulating—in other words, accumulation in a circular pattern around the outside thereof—occurs. And when PM burning starts in such a case during forced regeneration carried out after uneven accumulating, this uneven PM accumulation is all burned at essentially the same time and in a rapidly expanding pattern. As a result, extremely high temperatures occur within the interior of the filter. And this runaway burn is a cause of the DPF melting.
The problem of oil dilution becomes less frequent when a sufficiently long distance is traveled as fuel mixed into the oil evaporates. Accordingly, a combination of forced regeneration by multi-step injection and post injection is adopted for forced regeneration in response to a manual regeneration switch being operated when the travel distance has exceeded a predetermined travel distance and the exhaust gas temperature is low even if the vehicle is being currently traveled.
Furthermore, in the case of vehicles often driven by low-speed/high-load traveling patterns, flashing of the lamp to notify the driver the need of manual regeneration is so frequent that the driver feels it troublesome. In order to avoid this problem, the low collecting exhaust gas temperature raising control is carried out to raise the exhaust gas temperature by temporary control of fuel injection into the cylinder without an Unburned fuel supply control by post injection—the vehicle can be traveled until it has reached the distance at which the fuel mixed into the oil evaporates and regeneration during traveling is possible.
A first temperature raising judgment collecting quantity, which is less than the judgment collecting quantity corresponding to the notification of a manual regeneration request, and a second temperature raising judgment collecting quantity, which is less than the first temperature raising judgment collecting quantity, are provided for this low collecting quantity-time exhaust gas temperature raising control. The low-collecting quantity-time exhaust gas temperature raising control operates when the detected collecting quantity has exceeded the first temperature raising judgment collecting quantity, and thereafter, it is interrupted when the detected collecting quantity has fallen below the second temperature raising judgment collecting quantity.
Nevertheless, vehicles are actually traveled in a wide range of different traveling conditions. In particular, in such a place like an urban area, traveling and stopping are frequently repeated because of traffic signals and the like. This repetition makes engine load varies between traveling condition and stationary idling condition in a complicated manner. Consequently, the exhaust gas temperature also varies in a complicated manner.
In other words, in the case low collecting quantity-time exhaust gas temperature raising control is carried out using multi injection, when engine condition changes from traveling condition to stationary idling condition, the exhaust gas temperature raised during traveling of the vehicle drops as a result of factors such as reduced flow of exhaust gas when the vehicle stops. Furthermore, since the engine load in stationary idling is low, the combustion condition upon multi injection is not stable when low collecting quantity-time exhaust gas temperature raising control continues to be active, and as a result, exhaust gas temperature raising is not possible.
For this reason, different traveling patterns can cause problems such that unburned fuel ultimately turns into white smoke to be discharged to the atmosphere and fuel efficiency deteriorates. That is to say, it is possible that DPF regenerating control will not terminate within the preset time, that it will not be possible to sufficiently raise the exhaust gas temperature, and that the collected PM will not be burned and removed sufficiently. Furthermore, since the combustion condition is not stable, not all of the injected fuel is burned within each cylinder, and in addition, since the exhaust gas temperature is low, this unburned fuel is not oxidized by the action of the oxidation catalyst and flows therefrom into exhaust duct can occur.
In addition, as the exhaust gas temperature drops while the vehicle is in the stationary idling condition, even when traveling thereof restarts, a certain period of time is required until the exhaust gas temperature is raised to where self-regeneration of the DPF is accelerated, the time required for execution of low collecting quantity-time exhaust gas temperature raising control using multi injection becomes longer, and this causes a problem of impaired fuel efficiency.